E. R. F. Ramos

Collective excitation of a Bose-Einstein condensate by modulation of the atomic scattering length

We excite the lowest-lying quadrupole mode of a Bose-Einstein condensate by modulating the atomic scattering length via a Feshbach resonance. Excitation occurs at various modulation frequencies, and resonances located at the natural quadrupole frequency of the condensate and at the first harmonic are observed. We also investigate the amplitude of the excited mode as a function of modulation depth. Numerical simulations based on a variational calculation agree with our experimental results and provide insight into the observed behavior.

Observation of vortex formation in an oscillating trapped Bose-Einstein condensate

We report on the observation of vortex formation in a Bose-Einstein condensate of

Generation of nonground-state Bose-Einstein condensates by modulating atomic interactions

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Finite temperature correction to the Thomas–Fermi approximation for a Bose–Einstein condensate: comparison between theory and experiment

atoms. Vortices are generated by superimposing an oscillating excitation to the trapping potential introduced by an external magnetic field. For small amplitudes of the external excitation field we observe a bending of the cloud axis. Increasing the amplitude we observe formation of a growing number of vortices in the sample. Shot-to-shot variations in both vortex number and position within the condensed cloud are observed, probably due to the intrinsic vortex nucleation dynamics. We discuss the possible formation of vortices and antivortices in the sample as well as possible mechanisms for vortex nucleation.

Order parameter for the dynamical phase transition in Bose-Einstein condensates with topological modes

A technique is proposed for creating nonground-state Bose-Einstein condensates in a trapping potential by means of the temporal modulation of atomic interactions. Applying a time-dependent spatially homogeneous magnetic field modifies the atomic scattering length. An alternating modulation of the scattering length excites the condensate, which, under special conditions, can be transferred to an excited nonlinear coherent mode. It is shown that there occurs a phase-transition-like behavior in the time-averaged population imbalance between the ground and excited states. The application of the suggested technique to realistic experimental conditions is analyzed and it is shown that the considered effect can be realized for experimentally available condensates.

Ramsey-like fringes observation during excitation of coherent modes in a Bose-Einstein condensate

We observe experimentally a deviation of the radius of a Bose–Einstein condensate from the standard Thomas–Fermi prediction, after free expansion, as a function of temperature. A modified Hartree–Fock model is used to explain the observations, mainly based on the influence of the thermal cloud on the condensate cloud.

Evaporation in atomic traps: A simple approach

In a trapped Bose-Einstein condensate, subject to the action of an alternating external field, coherent topological modes can be resonantly excited. Depending on the amplitude of the external field and detuning parameter, there are two principally different regimes of motion, with mode locking and without it. The change of the dynamic regime corresponds to a dynamic phase transition. This transition can be characterized by an effective order parameter defined as the difference between fractional mode populations averaged over the temporal period of oscillations. The behavior of this order parameter, as a function of detuning, pumping amplitude, and atomic interactions is carefully analyzed. Special attention is paid to numerical calculations for the realistic case of a quadrupole exciting field and the system parameters accessible in current experiments.

Introduction to the Basic-Concepts of Bose-Einstein Condensation

In this work we analyse the excitation of coherent modes for different modalities for the time domain application of the external excitation. We have employed a single pulse excitation or a double pulse excitation. The population dynamic was analysed as a function of the frequency detuning between the modes energy separation and the external field frequency. For a single pulse, a Rabi like resonance profile was observed with maximum population transfer off resonance due to the strong non-linearity in the system. Line-shape as well as amplitudes are analysed. For a double pulse excitation, the occurrence of probbability interference generates a Ramsey-fringe like resonance with a considerable narrowing of the central line. Such profile has been investigated as a function of the delay between pulses revealing a dependence of the fringes distribution. It is found that the Ramsey pattern itself retains information about the accumulated relative phase between both ground and excited coherent modes.

Evaporação em armadilhas atômicas e as temperaturas mais baixas do Universo

We present a simple model to describe evaporative cooling of ultracold trapped atoms. The cooling is used to achieve low temperatures of the gaseous sample by removing the most energetic atoms and subsequently rethermalizing the remaining atoms at a lower temperature. The model assumes a Maxwell-Boltzmann distribution of energies and allows the calculation of the temperature and the number of remaining atoms after the most energetic atoms have been removed and rethermalization has taken place.

Experiments with Bose‐Einstein Condensation of Na and Rb

This text corresponds to part of the course presented at the ELAF XXXVIII. It is composed of work previously done along the years. The level of this introduction follows the last year of undergraduate and first year of graduate courses. We apologize to the experts on this field, but the idea here is to provide the basic principles and tools for students just getting involved with this topic. For those interested in a deeper reading on the subject, we strongly recommend the review article published by Courteille, Bagnato and Yukalov [1] from which part of this text was extracted. The lecture is divided in three parts: an introduction, the basic concepts of Bose‐Einstein Condensation and information about making and probing BECs.